Prevalence of the immune evasion gene cluster in Staphylococcus aureus CC398

Area of Science:

• Microbiology
• Genetics
• Epidemiology

Background:

• The immune evasion gene cluster (IEC) is characteristic of human Staphylococcus aureus strains.
• Livestock-associated methicillin-resistant S. aureus (LA-MRSA) CC398, often derived from human-adapted ancestors, typically lacks the IEC.
• Understanding IEC presence in animal-associated and human-infecting S. aureus is crucial for tracking strain evolution and host adaptation.

Purpose of the Study:

• To investigate the prevalence of the IEC in MRSA CC398 isolates from pigs, horses, and humans with occupational animal contact.
• To compare IEC presence in MRSA CC398 with methicillin-susceptible S. aureus (MSSA) CC398 isolates from human infections.
• To assess the implications of IEC acquisition and loss in the context of S. aureus host adaptation and evolution.

Main Methods:

• Collection and characterization of S. aureus isolates from various sources: pig nasal colonization, horse infections, human nasal colonization (pig farmers), veterinarian infections, and human infections.
• Detection of the immune evasion gene cluster (IEC) using molecular methods.
• Identification of specific Staphylococcus aureus CC398 subpopulations using PCR for canonical single nucleotide polymorphisms (SNPs) and detection of luk-PV.
• Comparative analysis of IEC presence across different host origins and S. aureus subtypes (MRSA vs. MSSA).

Main Results:

• The IEC was absent in pig nasal isolates but detected in some horse infections and veterinarians.
• MRSA CC398 isolates from pig farmers were negative for IEC, while some veterinarians had positive isolates.
• Among human MRSA CC398 infections, 19% carried the IEC, with a subset linked to the ancestral human-adapted subpopulation.
• A significant proportion of LA-MRSA CC398 from human infections had re-acquired the IEC, not always linked to luk-PV presence.
• MSSA CC398 isolates from human infections frequently contained the IEC, with some linked to the animal-associated subpopulation.

Conclusions:

• The presence of the IEC in animal-associated S. aureus and its re-acquisition by LA-MRSA CC398 suggests potential readaptation to the human host.
• IEC acquisition is not essential for LA-MRSA CC398 to cause infections in humans.
• Distinguishing between the ancestral human-adapted and animal-associated subpopulations of CC398 is critical for accurate epidemiological surveillance.